Like tile, most paint will differ slightly in each lot produced. If you plan to use the same color thoughout your house, it's a good idea to purchase as much paint as you can at one time. This will avoid any deviations in the paint color and save you an extra trip to the store.
Instead of putting a paint smear across the outside of a can to mark the level of paint left inside (which can be messy and obliterate the label), use a rubber band instead. Put it around the can and roll it down to the level of the paint. Just by glancing at the rubber band, which is visible all the way around, you will know how much paint is left.